CD8+ cytotoxic T lymphocytes (CTL) are critical effectors in the destruction of malignant and virus-infected cells, but they also mediate the rejection of transplanted organs and graft-versus-host disease in allogeneic bone marrow transplantation. Therefore, it is important to understand how CD8+ T cell work, in order to turn them on against viruses and tumors, and off against transplanted tissue. However, this understanding has remained elusive, in part because of the many variables which affect the on/off decision of a CD8+ T cell upon encountering antigen. These variables include the state of the CD8+ T cell (naive vs. memory), the type of antigen-presenting cell (APC), the nature of the antigen, and the presence or absence of "help" for the CD8+ T cell provided by CD4+ T cells. The broad aim of this project is to understand the cellar mechanisms of activation in mature CD8+ T cells, and the primary model system to be employed is the in vivo response of normal or H-Y receptor transgenic CD8+ T cells to the male-specific antigen, H-Y. This system offers three particular advantages: 1) the state of the responding cells may be controlled by using unprimed or H-Y primed females; 2) the fate of antigen-specific transgenic T cells may be tracked; and 3) the effects of different APC type may be determined by purifying the desired APC from male spleens and injecting them into syngeneic females. The hypotheses which serve as the basis for the experiments to be described are as follows: 1) Antigen presentation by "professional" APCs (such as dendritic cells) is both necessary and sufficient for the activation of naive CD8+ T cells. 2) CD4+ T cell help is both necessary and sufficient for the activation of memory CD8+ T cells responding to antigen on a "nonprofessional" APC, such as a B cell. The specific aims of the project are to: 1) Determine the APC requirements for the activation of naive CD8+ T cells. 2) Determine the role of CD4+ T cell help in the activation of naive CD8+ T cells. 3) Define the mechanism of tolerance induced by B cell antigen presentation to naive CD8+ T cells. 4) Understand the differences in kinetics and tolerance susceptibility of the in vivo responses to H-Y vs. multiple minor histocompatibility antigens. 5) Elucidate the molecular mechanisms of CD4+ T cell help for CD8+ CTL.